Piezoelectric ceramics are a type of piezoelectric material that converts electrical energy to mechanical energy, and mechanical energy to electrical energy, and they are applied in many electronic devices. Among such piezoelectric ceramics, binary Pb-containing piezoelectric ceramics constituted by the two constituents of PbTiO3 and PbZrO3 (hereinafter referred to as “PZT”) and tertiary Pb-containing piezoelectric ceramics containing Pb(Mg1/3Nb2/3)O3 or Pb(Zn1/3Nb2/3)O3 as the third constituent, are known. Such PZT provides high piezoelectric effect and is widely used as piezoelectric ceramic components. However, piezoelectric ceramics whose main constituent is PZT present problems in that they have high environmental burdens in the form of volatilization of PbO in the production process, flow-out of Pb constituent due to exposure to acid rain, and the like.
Accordingly, non-Pb piezoelectric ceramics containing no lead are pursued. As disclosed examples of Pb-free piezoelectric ceramics, Nature, 432 (4), 2004, pp. 84-87 (Non-patent Literature 1) and Applied Physics Letters 85 (18), 2004, pp. 4121-41232 (Non-patent Literature 2) disclose piezoelectric ceramics providing piezoelectric effect equivalent to PZT, having an alkali-containing niobate perovskite structure.
Such piezoelectric ceramics having an alkali-containing niobate perovskite structure are also disclosed in Japanese Patent Laid-open No. 2002-068835 (Patent Literature 1), Japanese Patent Laid-open No. 2003-342069 (Patent Literature 2), and Japanese Patent Laid-open No. 2004-300012 (Patent Literature 3). The piezoelectric ceramics in Patent Literatures 2 and 3 have Li, Na, K, Nb, Ta, Sb and O as main constituents, are expressed by the general formula {Lix[Na1-yKy]1-x}a{Nb1-z-wTazSbw}bO3 (in the formula, x, y, z, w, a and b indicate mol ratios, where 0≦x≦0.2, 0≦y≦1, 0≦z≦0.4, 0≦w≦0.2, a≦0.95, and b≦1.05), and have high piezoelectric characteristics (piezoelectric constant, electro-mechanical coupling coefficient, etc.). Also, as disclosed in Japanese Patent Laid-open No. 2008-169113 (Patent Literature 4), adding 0.001 mol to 0.15 mol of Ag as an additive element to 1 mol of chemical compound whose main constituent is expressed by the above general formula will improve at least one of the characteristics that include the piezoelectric constant, electro-mechanical coupling coefficient, specific dielectric constant, dielectric loss, and Curie temperature.
Among the piezoelectric ceramics expressed by the above general formula, however, antimony (Sb) belongs to the heavy metal class and presents concerns over toxicity to the human body. Accordingly, piezoelectric ceramics not containing antimony are desired. Also, while tantalum (Ta) can lower the Curie temperature, raise the dielectric constant of piezoelectric ceramics, and enhance the piezoelectric constant and other characteristics, use of tantalum raises the sintering temperature needed to obtain dense ceramics.
As explained above, piezoelectric ceramics having an alkali-containing niobate perovskite structure, not containing Sb or Ta and having Li, Na, K, Nb and O as constituent elements (hereinafter referred to as “LNKN”), offer excellent piezoelectric characteristics, but the sintering property is poorer than Pb-containing piezoelectric ceramics. For example, Li0.06Na0.47K0.47NbO3, which is a type of LNKN, exhibits excellent characteristics as evident from displacement characteristics d33 in the polarization axis direction of 235 pC/N, but it must be sintered at temperatures of 1050° C. to 1100° C. in order to obtain a dense sintered compact, as disclosed in Non-patent Literature 2. Sintering at high temperatures causes alkali metal elements to evaporate easily and piezoelectric effect may deteriorate. Accordingly, sintering temperatures must be controlled precisely in order to obtain ceramics having sufficiently high piezoelectric effect.
Japanese Patent Laid-open No. 2008-207999 (Patent Literature 5) discloses a technology to obtain a dense sintered compact at low sintering temperatures by using a sintering auxiliary in which Li2CO3, LiBO2 and Li2B4O7 are mixed. However, Li2CO3 remains in the sintered compact as Li2O after sintering, which causes the resistance of ceramics to drop. Additionally, LiBO2 and Li2B4O7 can cause piezoelectric characteristics to drop.
Furthermore, Japanese Patent Laid-open No. 2004-115293 (Patent Literature 6) discloses a piezoelectric ceramic whose sintering property has been improved by adjusting the composition of the starting material to contain excessive elements (Nb, Ta) at the B site of (K,Na)(Nb,Ta)O3 beyond the stoichiometric ratio, and also by adding CuO. However, this piezoelectric ceramic also requires sintering temperatures of 1050° C. or above in order to obtain a dense sintered compact.